K327 / K328 / 2027

Lesson 5 of 7 / Nutrition in humans

Hormones carry chemical messages

How can a gland affect an organ far away?

I can define a hormone by its production, blood transport and effects on specific target organs.

  • I can define a hormone.Syllabus

    Syllabus K327 / K328, Topic 4(e). Define a hormone.

About 4 min

Make a guess. You are not marked.

Insulin travels in the blood to every organ in your body. Does every organ respond to it?

  1. Yes. Anything the blood reaches must respond.
  2. No. Insulin only travels to the liver through a duct.
  3. No. Only target cells with the right receptors respond.
Show the answer

No. Only target cells with the right receptors respond.

A hormone reaches everywhere but only acts where cells have matching receptors.

The key idea

A hormone is a chemical substance produced by a gland, carried in the blood and affecting specific target organs.

Explore the idea

Which route carries the message?

Pancreas makes insulinInsulin enters bloodSpecific targets respondGlucose uptake and storage

A gland makes a hormone, blood carries it and specific target cells change their activity. Insulin is a signal, not the glucose being stored.

Arrows show a causal sequence. They do not mean that a hormone turns into the substance named in the next box.

Explanation

A hormone is a chemical substance produced by a gland, carried by the blood and changing the activity of one or more specific target organs. The gland and the target do not have to be beside each other because circulation connects them.

Blood carries a hormone past many tissues, but only suitable target cells respond. Their receptors allow them to recognise the signal. Transport around the body does not mean that every organ responds in the same way.

The pancreas produces insulin and glucagon and releases them into the blood. These hormones help regulate blood glucose. The pancreas also releases digestive enzymes into the small intestine through a duct. A hormone in blood and a digestive enzyme in the gut have different routes and jobs.

A hormone is a signal that changes activity. Insulin does not become glycogen or digest glucose. It promotes changes in target cells that affect glucose uptake and storage. This distinction lets us explain control without confusing the signal with the substance being controlled.

Step by step
  1. 1

    Name the source

    State that a gland produces the hormone.

  2. 2

    Follow the route

    Blood carries the hormone to its targets.

  3. 3

    State specificity

    Only suitable target organs change activity in response.

Worked example

Two pancreatic secretions

Insulin enters blood while pancreatic amylase enters the intestine. Explain which is a hormone and why.

One way to explain it

Insulin is a hormone: it is produced by the pancreas, carried in blood and changes the activity of specific target cells. Amylase is a digestive enzyme that acts on starch in the gut.

Why this answer works
  • Use all parts of the hormone definition.
  • Compare transport routes.
  • Separate signalling from digestion.

Watch out for this

A student says: "Every organ responds equally because the blood carries a hormone everywhere." What is wrong with this?

Show the answer

A hormone affects suitable target cells with the appropriate receptors. Circulation and response are different things.

Check your understanding

Which description contains all parts of the hormone definition?

  1. A chemical made by a gland, carried by blood and altering specific target-organ activity.
  2. An enzyme made by a gland and carried down a duct to the gut, where it digests food.
  3. Any substance in the blood that supplies energy.
Show the answer

A chemical made by a gland, carried by blood and altering specific target-organ activity.

Production, transport and target effect are all included.

Quick recall

Card 1 of 3. Answer in your head, then check.

How can a gland affect an organ far away?

You can return to this lesson any time.